ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-11-767-2011Atmospheric ions and nucleation: a review of observationsHirsikkoA.1NieminenT.1GagnéS.12LehtipaloK.1ManninenH. E.1EhnM.1HõrrakU.3KerminenV.-M.14LaaksoL.145McMurryP. H.6MirmeA.3MirmeS.3PetäjäT.1TammetH.3VakkariV.1VanaM.13KulmalaM.11Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland2Helsinki Institute of Physics and University of Helsinki, Department of Physics, P.O. Box 64, 00014 University of Helsinki, Finland3Institute of Physics, University of Tartu, 18 Ülikooli Str., 50090 Tartu, Estonia4Finnish Meteorological Institute, Research and Development, P.O. Box 503, 00101 Helsinki, Finland5School of Physical and Chemical Sciences, North-West University, Potchestroom, Republic of South Africa6Particle Technology Laboratory, University of Minnesota, Minneapolis, Minnesota, USA26012011112767798This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://www.atmos-chem-phys.net/11/767/2011/acp-11-767-2011.htmlThe full text article is available as a PDF file from https://www.atmos-chem-phys.net/11/767/2011/acp-11-767-2011.pdf

This review is based on ca. 260 publications, 93 of which included data on
the temporal and spatial variation of the concentration of small ions (<1.6 nm in diameter) especially in the lower troposphere, chemical
composition, or formation and growth rates of sub-3 nm ions. This
information was collected on tables and figures. The small ions exist all
the time in the atmosphere, and the average concentrations of positive and
negative small ions are typically 200–2500 cm<sup>−3</sup>. However,
concentrations up to 5000 cm<sup>−3</sup> have been observed. The results are in
agreement with observations of ion production rates in the atmosphere. We
also summarised observations on the conversion of small ions to intermediate
ions, which can act as embryos for new atmospheric aerosol particles. Those
observations include the formation rates (<i>J</i><sub>2</sub>[ion]) of 2-nm intermediate
ions, growth rates (GR[ion]) of sub-3 nm ions, and information on the
chemical composition of the ions. Unfortunately, there were only a few
studies which presented <i>J</i><sub>2</sub>[ion] and GR[ion]. Based on the publications,
the formation rates of 2-nm ions were 0–1.1 cm<sup>−3</sup> s<sup>−1</sup>, while the
total 2-nm particle formation rates varied between 0.001 and 60 cm<sup>−3</sup> s<sup>−1</sup>. Due to small changes in <i>J</i><sub>2</sub>[ion], the relative importance of ions in 2-nm particle formation was determined by the large
changes in <i>J</i><sub>2</sub>[tot], and, accordingly the contribution of ions increased
with decreasing <i>J</i><sub>2</sub>[tot]. Furthermore, small ions were observed to
activate for growth earlier than neutral nanometer-sized particles and at
lower saturation ratio of condensing vapours.